Ken,
My understanding is that the nulls of a beam antenna are not
particularly dependable. Most beams were designed for gain, not a
rock steady null off the back and/or sides. They can move around,
depending on frequency, and environment around the antenna. Perhaps
better to look for maximum signal, not minimum.
On the other hand, small bidirectional loops do tend to have good
predictable nulls, and many of them are designed to preserve that.
There's also some chance the power lines might be conducting the noise
away from its source and radiating it some distance away, so you might
triangulate on what happens to be an efficient radiator rather than
the source of the noise.
If you do have standing waves on your feedlines ... actually not that,
but common mode currents ... say if you didn't use a balun when making
a transition between unbalanced and balanced ... then your feedline is
an unintended antenna element and could skew your antenna's patterns
significantly.
Regards,
Andy
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